This invention relates to a circuit arrangement for operating a discharge lamp, comprising
a load branch B provided with lamp connection terminals,
a DC-AC converter provided with a branch A coupled to the load branch B and comprising at least one switching element for generating a current of alternating polarity through the load branch B by being alternately conducting and non-conducting at a frequency f,
a drive circuit E for rendering the switching element alternatively conducting and non-conducting at the frequency f,
a control circuit C coupled to the drive circuit and the discharge lamp for generating a control signal which is dependent on the lamp current and serves to influence the frequency.
Such a circuit arrangement is described in the European Patent Application EPA 0351012.
The circuit arrangement described therein controls the amplitude of the lamp current of a discharge lamp operated by the circuit arrangement at a substantially constant level.
If the control signal is also dependent on the lamp voltage, it is possible to control the average value of the power consumed by the lamp (this average value will be called the lamp power hereinafter) at a substantially constant value for various types of discharge lamps and to render it substantially independent of factors such as variations in the supply voltage or fluctuations in the ambient temperature. If the control signal is dependent on a desired average value of the power consumed by the discharge lamp, there is a possibility of dimming the discharge lamp through adjustment of the desired average value of the power consumed by the discharge lamp. When the setting of the desired average value of the power consumed by the discharge lamp is changed, the value of the frequency f is adapted in such a way that the lamp power is substantially equal to the desired power. This adjustment possibility for the lamp power, however, functions only over a lamp power range within which there is an unequivocal relation between the lamp power and the frequency f. Every value of the frequency f in that case corresponds to one value of the lamp power. Since the load branch B often comprises inductive means connected in series with the lamp, the lamp power decreases with an increase in the frequency f. Such a relation is found over a comparatively wide lamp power range in practice for many discharge lamps of various types and power ratings. This relation renders it possible to adjust the lamp power over a desired range by means of the frequency f.
For some discharge lamps, however, the relation between the frequency f and the lamp power is not unequivocal over a part of a desired adjustment range of the lamp power. As a result, there is also no unequivocal relation between the control signal and the lamp power over this portion of the desired adjustment range of the lamp power. It is found for certain compact fluorescent lamps, for example, that the lamp power increases with an increase in the value of the frequency f over a certain lamp power range, whereas the lamp power decreases with an increasing frequency f for lamp power values outside this range. This means that, within a certain range of the frequency f, every value of the frequency f corresponds to two or more different values of the lamp power. These lamp power values also fail to show an unequivocal relation to the control signal. Lamp power values situated within the range over which the lamp power increases as a function of the frequency cannot be adjusted: an oscillation of the lamp power is found to take place between the desired value and a second value of the lamp power belonging to the relevant value of the frequency f. Besides a relation between lamp power and the frequency f within a certain lamp power range which is not unequivocal, there is also found to exist a relation between the average lamp current and the frequency f within a certain range of the average lamp current which is not unequivocal for such lamps. The result is that some values of the average lamp current cannot be adjusted, while for some settings oscillations in the lamp current amplitude are found to occur.